This invention relates to a method of making 2,4,5-trihalobenzoic acid from 2,4,5-trihalo-N-alkyl or aryl phthalimide. In particular, it relates to a method in which 3,4,6-trihalo-N-alkyl or aryl phthalimide is heated to a temperature of about 130.degree. to about 190.degree. C. in the presence of water and a mineral acid to produce 2,4,5-trihalobenzoic acid in a high yield.
2,4,5-trifluorobenzoic acid, a useful intermediate for manufacturing quinolone antibacterial drugs, can be prepared by selective decarboxylation of trifluorophthalic acid. However, preparation of trifluorophthalic acid from inexpensive trichlorophthalic or tetrachlorophthalic anhydride, via by direct fluorination using KF treatment is difficult because of breakage of carboxylic groups under the harsh fluorination conditions.